Modern telecommunication systems are equipped to reliably handle a variety of telecommunications traffic. In one arrangement, known as the Integrated Services Digital Network (ISDN) both voice and data are switched in digital form. Call signaling information such as origination signaling, dialed digits, etc. is also transmitted in digital form. Data packets containing signaling information or data are transmitted between a subscriber terminal and the switching system via a digital subscriber link, usually over designated channels as specified for ISDN by the International Telegraph and Telephone Consultative Committee (CCITT). Within the switching system, incoming packets are routed to packet switching devices known as protocol handlers which route data packets to circuits that communicate with corresponding peer protocol circuits in the subscriber terminals, using protocols defined for ISDN.
High reliability is customarily achieved in switching system designs by duplicating critical elements. However, ISDN systems will normally include a substantial number of protocol handlers which are complex and expensive units making full duplication of these units very expensive. As an alternative to full duplication, an n+1 sparing design approach has been used in other systems in which one unit (e.g., a time-slot interchange unit or a memory unit) is designated as the spare for N associated active units. With such a design approach, the spare unit is provided with special connections to the associated units and can be controlled to temporarily perform the functions of one of the associated units in case of failure. Such an arrangement is cheaper than full duplication but provides a lesser degree of reliability. For some telecommunications applications, however, a lesser degree of reliability is acceptable. For example, in data communications, lower reliability of the switching equipment can often be tolerated because of extensive error checking and the ability to retransmit incorrectly received data. On the other hand, very high reliability may be required in the case of military voice communications or similar applications where emergency situations may occur. Furthermore, both types of communications, some requiring high reliability and others requiring a lesser degree of reliability, may have to be handled in one switching system. A problem of the prior art is that there is no convenient way of providing spare units, such as protocol handlers, in such a way that the system may be adapted for a variety of reliability needs without a redesign and restructuring of the system.